With the rapid development of wireless communication technology, communication systems are also evolving, employing it. One of the examples is a long term evolution (LTE) system, which has been developed as the 4th generation LTE system. LTE systems employ a variety of technologies to meet the rapid increase in traffic demand, one of which is carrier aggregation. Carrier aggregation refers to a technology that increases the number of carriers in communication between user equipment (UE) and an evolved Node B (ENB) to employ from one carrier in conventional art to a primary carrier and one or more secondary carriers, thereby increasing the amount of transmission by the number of secondary carriers. In LTE technology, a cell where a primary carrier is served is called a Primary Cell (PCell) and a cell where a secondary carrier is served is called a Secondary Cell (SCell).
In order to comply with the rapid increase in traffic, service providers have installed Pico ENBs with narrow service coverage as well as Macro ENBs with wide service coverage. This causes overlaps between the service coverages of Pico ENB and Macro ENB. In order to increase data transmission rate of UE in a state where the service coverage overlap occurs, a technology has been discussed where UE are simultaneously connected to a macro ENB and a pico ENB and also uses the carriers of the Macro ENB and carriers of the Pico ENB.
In the scenario described above, a macro ENB has three carriers (since a primary ENB doesn't have to be a macro ENB, it is hereinafter called a serving ENB). UE uses one of the three carriers as a PCell and the two carriers as SCell 2 and SCell 2 respectively. In addition, a pico ENB has two carriers (since the other ENBs except for the primary ENB don't have to be pico ENBs, they are hereinafter called drift ENBs). UE uses one of the three carriers as a PCell and the two carriers as SCell 2 and SCell 2 respectively. UE uses the two carriers as SCell 3 and SCell 4. Carriers (PCell, SCell 1 and SCell 2) of the serving ENB are called a Primary Set. Carriers (SCell 3 and SCell 4) of the drift ENB are called a Non-primary Set.
UE transmits a Buffer Status Report (BSR) on the uplink via a cell in order to report an amount of data to be transmitted. Since the UE is connected with a number of ENBs, there arises the problem of how many resources each ENB must allocate to the UE.